Thermal absorber coatings, which are produced by vacuum deposition techniques on plastic or metal substrates, have a wide potential application due to their high solar absorbance in combination with a low thermal emittance at operation temperatures. Simultaneously, the coatings have good adherence on a substrate, high hardness, and corrosion resistance.
The thermal absorber coatings are widely mentioned in the literature and those are used in industrial applications mainly to harvest thermal energy from the solar radiation so that the coatings are deposited on a tubular or flat substrate, which is integrated into a solar absorber element.
Typical operation temperatures of the solar absorber element range from about 100° C. in low temperature collectors as they are found in household applications, 200° C. to 500° C. in collector elements for a production of a process heat to up to 1200° C. in solar towers for an electrical energy production.
A coating may consist in a simple black organic paint, a monolitic metallic, ceramic, or organic coating, or a multilayered optical stack. An optical performance of the coatings is expressed by an optical absorbance of the coating a in a wavelength range relevant for the solar radiation and a radiative thermal emission ε in a wavelength range relevant for the black body radiation at an operating temperature. The best optical performance is achieved by multilayered optical stacks. The multilayer stacks can have a value of α>92% and a value of ε<10% at temperatures below 200° C. (the monolitic coatings exhibit typically an emission value >10%).
One example of a commercially introduced multilayer stack consists of a layer having titanium and nitrogen (TiN), a layer having titanium, nitrogen, and oxygen (TiNxOy), and a layer having silicon and oxygen (SiOx). This multilayer stack having the aforesaid layer compositions is claimed to achieve the values of α>95% and ε<5% with a maximum operation temperature of up to 200° C. Other solutions, primarily in an attempt to extend the operation temperature and a lifetime of a solar absorber element, show very similar coating compositions, replacing the metal in the above mentioned coatings by chromium, aluminium, niobium, and a composition of aluminium and titanium (AlTi), but the optical performance of these coatings does not improve.
The coatings degradate generally by three basic processes. Oxygen or other corrosive elements from the atmosphere corrode the top layer of the coatings, or permeate towards a second or third coating layer and cause a decoloration of the layer, generally resulting in a reduction of α and an increase of ε. Secondly, elements from a substrate material, especially in the case of copper substrates, diffuse into the coating causing again the decoloration. Thirdly, during the lifetime of a thermal absorber coating, such coating is exposed to numerous thermal cycles which may cause a loss of coating adherence and spallation that results in a complete failure of a product having the coating. The third degration process is, of course, accelerated by the above-mentioned processes.
These preceding drawbacks have limited significantly the usability of the existing thermal absorbers.